It is known to extract tungsten values from an aqueous phase by using a solvent extraction process in which the liquid phases involved are substantially immiscible. Solvent extraction is used to separate tungsten values from contaminants which accompany tungsten in solution in the aqueous phase during leaching of tungsten oxide concentrates. These contaminants include the sodium ion and the sulfate ion, and also include silica, fluorine, calcium and magnesium.
The tungsten-bearing aqueous phase is intimately mixed with an organic phase typically comprising at least one extractant for the tungsten values and one or more other liquid organic compounds. The tungsten values are extracted into the organic phase, and the mixture of organic and aqueous phases is allowed to settle so that the two phases can be recovered separately. The steps of mixing and settling can be repeated several times to improve the overall degree of extraction of tungsten values.
A problem encountered in solvent extraction of tungsten values is the formation of a third phase. The third phase can comprise liquid, solids, or a mixture of liquid and solid. The third phase is formed when the tungsten content extracted into the organic phase exceeds a moderate level. The third phase builds up, so that process equipment must periodically be shut down to remove it, and it represents lost extraction efficiency. This problem can be caused by the excessive transfer of impurities from the aqueous phase into the organic phase, or by the choice of constituents used in the organic phase. This drawback can require the operator to tolerate low tungsten contents in the organic phase so as to avoid formation of the third phase.
A related problem is the slow rate at which the aqueous and organic phases separate in the settling stage. Stated another way, the mixture of aqueous and organic phases fed to a settler should separate quickly into two discrete phases, but in actual practice relatively long periods of time may be necessary to separate the two phases to a satisfactory degree.
Phase disengagement rate is a critical property in solvent extraction. In the publication Separation Science And Technology, 16(9), pp. 1261-1289 [1981], phase disengagement is discussed in some detail. According to the article, the type of extractant used appears to have an effect on phase disengagement. Batch phase disengagement tests have shown significant trends with respect to the structure of the amine used as the extractant. In general, organic continuous phase disengagement became slower with increasing number of carbon atoms per chain, whether branched or linear chain. However, for any given number of carbons, the branched chain amines had a much faster organic continuous disengagement than the linear chains.
The choice of phase continuity is important in certain solvent extraction operations. For example, in stripping tungsten from a loaded organic using ammonium hydroxide as the stripping solution, an aqueous continuous mixture of aqueous and organic solutions is preferred. During stripping it is important that precipitation of ammonium paratungstate (APT) be prevented. This is accomplished by injecting the tungsten-loaded organic into a high-shear region near the impeller of the strip mixer. The mixer is operated with the aqueous phase continuous in order to facilitate the rapid pH change required to prevent APT from precipitating. However, by operating the mixer aqueous-continuous, the entrainment of organic in the aqueous solution can be substantial, the solution thereafter going to the APT crystallizer. A gradual build-up of organics (total organic carbon, TOC) through recycling of the crystallizer mother liquor results in foaming in the crystallizer which is not desirable. Moreover the disengagement of the phases is not all that is desired.
Recently, it has been determined that agitation and rapid mixing of the organic and aqueous phases is more important than phase continuity. However, the type of phase continuity itself can be important to assure efficient disengagement of the phases in the settler.
It would be desirable to provide a solvent extraction process in which entrainment of the organic in the aqueous phase is substantially reduced and the rate of disengagement between the two phases markedly increased.